Dunite formation processes in highly depleted peridotite: Case study of the Iwanaidake peridotite, Hokkaido, Japan

Dunite formation processes in highly depleted peridotites are discussed based upon a detailed study of the Iwanaidake peridotite, Hokkaido, Japan, which consists mainly of harzburgite with a small amount of dunite. In the harzburgites, the Mg# [= 100 x Mg/(Mg + Fe2+)] of olivine ranges from 91.5 to 92.5, and the Cr# [= 100 x Cr/(Cr + Al)] of spinel from 30 to 70; in the dunites, the Mg# of olivine ranges from 92.5 to 94 and the Cr# of spinel from 60 to 85,respectively. The NiO wt % of olivine in harzburgites ranges from 0.38 to 0.44, and in dunites from 0.35 to 0.37. The Mg# and Cr# are higher and NiO wt % is lower in the dunites than in the harzburgites surrounding the dunites. The Mg# and Cr# exhibit normal depletion trends expected from simple partial melting, whereas the NiO wt % shows an abnormal trend. On the basis of mass balance calculations, dunites are considered to be derived from the harzburgites by a process involving incongruent melting of orthopyroxene (orthopyroxene --> olivine + Si-rich melt). Hydrous conditions were necessary to lower the solidus, and thus melting of harzburgite alas probably triggered by the introduction of hydrous silicate melt. The dunite in this massif may have formed in the mantle wedge above a subduction zone.